Jenkins is a popular open-source platform used by millions of developers for continuous integration and build automation. Sometimes, third-party plugins add extra features to Jenkins. One such plugin is the SSH2 Easy Plugin—a handy tool for running remote jobs over SSH.

But in 2023, security researchers found a significant vulnerability, now tracked as CVE-2023-41939. Here’s what went wrong, why it matters, and how attackers could exploit it—explained in simple terms with a step-by-step demo.

What’s the Problem?

Jenkins plugins, including SSH2 Easy, let administrators say exactly which users can do what (these are called "permissions"). For example, you might have the powerful “Manage” permission or just the “Read” permission.

The bug here: The SSH2 Easy Plugin does not properly check if a permission supposedly granted is still really enabled for a user. That means, if you used to have a high-level permission and it was turned off for you later, you might still get access to stuff you’re not supposed to see or use anymore.

In Other Words

- You had permission to manage something (e.g., “Overall/Manage”).

Why Is This Dangerous?

If a user leaves a company or their role changes, they’re supposed to immediately lose sensitive controls. Otherwise, people could accidentally (or on purpose) break stuff, access sensitive systems, or introduce malicious code.

Who’s Affected?

- Jenkins servers using the SSH2 Easy Plugin.

Plugin version 1.4 and earlier.

- Anyone using fine-grained permission control in Jenkins, especially controlling “Overall/Manage” or similar admin roles.

Where’s the Official Info?

- Jenkins Security Advisory Archive
- NIST NVD Entry (CVE-2023-41939)
- Original SSH2 Easy Plugin

How Can Someone Exploit This? (Proof-of-Concept Walkthrough)

Let’s see what this looks like in the real world with a simple scenario.

1. Initial Setup

Suppose Alice is a Jenkins user. She gets the “Overall/Manage” permission.

// Jenkins security setting (admin-side)
grantPermission('alice', 'Overall/Manage')

She can use the SSH2 Easy Plugin to start/stop jobs, create jobs, etc.

Later, the admin removes her “Manage” powers

// Remove the powerful permission
revokePermission('alice', 'Overall/Manage')

But here’s the problem: in SSH2 Easy Plugin version 1.4 and earlier, the plugin does NOT double-check that Alice really lost her access before letting her keep controlling things!

Alice uses the plugin to run a “manage” operation, maybe to update jobs over SSH

# Alice (using SSH2 Easy functionality)
curl -u alice:herpassword -X POST \
  http://jenkins.example.com/plugin/ssh2easy/manageJob \
  -d "job=SensitiveBuild&action=delete"

Expected result: Alice should get an “access denied” error.

But with the vulnerable plugin: Alice’s request goes through—she still has manage privileges through the plugin, even though the main Jenkins UI knows she shouldn’t.

Relevant Plugin Check (Pseudo-code)

public boolean canManage(User user) {
    // Vulnerable logic: only checks a plugin config, NOT actual permission
    return pluginConfig.getEnabledPermissions().contains("Manage");
}

What it should do

public boolean canManage(User user) {
    // Secure logic: always check system permission, NOT just plugin config
    return jenkins.hasPermission(user, Permission.MANAGE);
}

Because it skips the real Jenkins permission check, old permissions linger.


## How to Fix / Mitigate

Patch Immediately:
Upgrade to SSH2 Easy Plugin 1.5 or newer. The fix now checks Jenkins’s own permissions before granting access.

Disable/Remove the Plugin:
If you can’t update now, consider disabling or removing the plugin until it’s safe.

Review All User Permissions:
Check for old users who had higher permissions and double-check your audit logs.

Final Thoughts

Privilege management is core to security. The CVE-2023-41939 bug in Jenkins SSH2 Easy Plugin shows that even seemingly small permission check mistakes can have big consequences. If you run Jenkins with this plugin, patch now!

References

- Jenkins Security Advisory – 2023-09-20 #SECURITY-3202
- NVD NIST Entry: CVE-2023-41939
- Jenkins SSH2 Easy Plugin Page

Timeline

Published on: 09/06/2023 13:15:00 UTC
Last modified on: 09/11/2023 17:51:00 UTC